This research is concerned with the study of primate vocalizations, with the goals of developing an understanding of the evolution of human speech and language and of developing animal models for the study of speech and language disorders. We seek to identify similarities in mechanisms of vocal communication between animals and human beings. We are studying the complexity of primate vocalizations, attempting to identify the various inputs that lead to the complexity and the nature of the mechanisms by which animals perceive and act upon these vocalizations. On the basis of vocal signals alone animals recognize subtle differences in a message communicated, recognize individuals, recognize members of their troops, and members of similar subspecies. Calls are varied to improve the perception of signals by other animals according to environmental conditions. The basic mechanisms of perception of vocalizations by monkeys are similar to those used by human beings to understand speech, and these mechanisms are different from those used to perceive non-communicatory sounds. Finally, there are parallels between the ontogeny of vocal communication in animals and human beings. This research program will show that animals do have some components of human language and will be useful models for the study of language disorders.